1. Field of the Invention
This invention relates to cycles for use in steam sterilization.
2. Description of the Prior Art
Various cycles have been designed for steam and ethylene oxide gas sterilization as it is used in hospitals and laboratories. These cycles may be characterized by the medium used; for example a steam cycle or ethylene oxide gas cycle; or by the nature of the fluid flow therein, such as a gravity cycle. Cycles in this art are also designated by the nature of the load; for example, a vacamatic cycle is one for a load of wrapped fabric goods such as bedding or linens; and a liquid cycle is used when liquids are being sterilized.
For descriptive purposes a steam or gas cycle may also be divided into four functional steps: (i) removing air from the chamber; (ii) removing air from the load and humidifying the load; (iii) exposing the load, or actual sterilization of the load and (iv) drying the load. In the art, step (ii) is also termed "conditioning" the load.
The conditioning phase, which removes air from the load and moisturizes the load, is particularly important to the sterilization process. Moist heat in the form of saturated steam kills microorganisms by thermal destruction and heat denaturation of microbial cell proteins. Direct steam contacts the cell, condensation occurs, and the latent heat of vaporization is discharged. When moisture is present, coagulation takes place at relatively lower temperatures; but when no moisture is present, higher temperatures are required. Hot air alone, not carrying this latent heat, is not an effective sterilant, and may also damage a load, such as fabric because of the high temperatures required. It is essential to effective sterilization that moist heat penetrate the load and contact all surfaces; any air present will impede sterilization. Thus it is critical to remove substantially all air from the chamber and the load, as well as to humidify and moisturize the load. Chamber air may generally be adequately removed by drawing a vacuum in the chamber (0-3/4 inch mercury). However it is more difficult to remove air from a load, particularly a wrapped fabric (vacamatic) load, because the air becomes trapped therein. Conditioning therefore becomes especially critical for vacamatic cycles.
A steam sterilization cycle is essentially a combination of the use of steam and/or vacuum in a pressure chamber to sterilize the load therein. It is known in steam sterilization cycles to use the gravity displacement of air in the chamber by steam for the aforesaid steps (i) and (ii). It is also known to use what is known in the art as "pulsing", which may be commonly employed for step (ii). Pulsing is the process of increasing chamber pressure to a maximum pressure, generally by admission of steam; followed by decreasing sterilizer pressure to a minimum pressure (by drawing a vacuum and/or shutting off steam) to a minimum pressure preceding the next pulse.
Pulsing has proven to be an effective means of conditioning a load for sterilization, however generally requires a high energy consumption in terms of steam requirements and the use of vacuum pumps. Gravity air displacement, on the other hand, is energy efficient; however it is prohibitively time consuming and does not remove chamber air as efficiently as a vacuum pump. Vacuum/steam cycles without pulsing do not always adequately condition a load for proper sterilization. While combinations of these processes have been employed in the art they have been subject to the aforesaid limitations; they either have a high steam consumption, or vacuum pump energy demand, require excessive time to complete the cycle, or result in inadequate sterilization for certain types of loads.